A display apparatus having a structure in which a liquid crystal panel is provided is conventionally well known as one type of matrix-type display apparatus. One example of such a display apparatus is one having a structure in which a dot-matrix-type display apparatus (for example, M-row/N-column active matrix liquid crystal panel) is assembled together with a liquid crystal panel for switching two-dimensional (2-D) display and three-dimensional (3-D) display (hereinafter, this kind of liquid crystal panel is referred to as a switching liquid crystal panel), and being able to electrically switching the two-dimensional display and the three-dimensional display (e.g. see Publication of Japanese Patent Application, Tokukaihei, publication No. 3-119889 (published on May 22, 1991).
FIGS. 12 and 13 illustrates cross sections of a panel section of the liquid crystal display apparatus for dual-display of two-dimensional and three-dimensional images. FIG. 12 illustrates the cross section of the panel section when the three-dimensional image is displayed. FIG. 13 illustrates the cross section of the panel section when the two-dimensional image is displayed. As illustrated in FIGS. 12 and 13, the liquid crystal display apparatus for dual-display of the two-dimensional image and the three-dimensional image is provided with a dot-matrix type liquid crystal panel 101 and a switching liquid crystal panel 102 assembled together.
The dot-matrix liquid crystal panel 101 is provided with a polarizer 111, a counter electrode 112, a liquid crystal layer 113, a pixel electrode 114, and a polarizer 115, which are arranged in lamination. The pixel electrode 114 is provided with an active element (not illustrated). The active element is turned ON/OFF in accordance with a scanning signal inputted from a scanning electrode control circuit (not illustrated). When the active element is ON, a video signal is inputted from a video signal control circuit (not illustrated) into the pixel electrode 114. In other words, the dot-matrix type liquid crystal panel 101 serves as display image generating means for generating a displayed screen in accordance with image data.
The switching liquid crystal panel 102 is provided with a patterned retardation film 121, an upper electrode 122, a liquid crystal layer 123, a lower electrode 124, and a polarizer 125, which are arranged in lamination. According to whether or not a voltage is applied on the liquid crystal layer 123, the switching liquid crystal panel 102 switches over whether a parallax barrier is present or absent (i.e. whether the parallax barrier is effectuated or not).
The upper electrode 122 and the lower electrode 124 are connected with a power source circuit (not illustrated) for applying a driving voltage on these electrodes. In accordance with whether the two-dimensional image or the three-dimensional image is to be displayed, the power source circuit switches over whether it supplies the voltage or not. For instance, the voltage applications switches over the display to the two-dimensional image display. On the other hand, when no voltage application is made, the display is switched over to the three-dimensional image display. In accordance with whether or not the voltage is applied on the upper electrode 122 and the lower electrode 124, the polarization state of light passing through the liquid crystal layer 123 is switched over.
The patterned retardation film 121 has two types of regions 121A and 121B, which are in a stripe shape and in alternative arrangement. The two types of regions 121A and 121B have different polarization states. The light having passed through the liquid crystal layer 123 enters the patterned retardation film 121. The regions 121A and 121B of the patterned retardation film 121 have different rubbing directions. In other words, their slow axes are in different directions. Therefore, light having passed through the region 121A and light having passed through the region 121B have different polarization state. For instance, it is designed that a polarization axis of the light having passed through the region 121A and that of the light having passed through the region 121B make 90 degrees.
The polarization states of the light having passed through the regions 121A and 121B depend on the polarization state of the light incident on the patterned retardation film 121, that is, the polarization state of the light having passed through the liquid crystal layer 123. The light having passed through the patterned retardation film 121 enter the polarizer 115 of the dot-matrix type liquid crystal panel 101.
When the image is displayed as a three-dimensional image, that is, when no voltage is applied on the liquid crystal layer 123, an optical axis of the light having passed through the region 121A is parallel with a transmission axis of the polarizer 115, but an optical axis of the light having passed through the region 121B is perpendicular to the transmission axis of the polarizer 115.
In other words, as illustrated in FIG. 12, the patterned retardation film 121, working together with the polarizer 115, causes optical effect thereby effectuating functions of a parallax barrier. The regions 121A of the patterned retardation film 121 servers as a transmission region, while the region 121B serves as a blocking region.
Light having passed through the region 121A and the polarizer 115 and traveling toward a right eye (i.e. in a right-eye direction) passes through that part of the liquid crystal layer 113 on which the voltage is applied by a pixel electrode 114R that is for displaying in accordance with image data for the right eye (hereinafter, this image data is referred to as right-eye image data). On the other hand, light having passed through the region 121A and the polarizer 115 and traveling toward a left eye (i.e. in a left-eye direction) passes through that part of the liquid crystal layer 113 on which the voltage is applied by a pixel electrode 114L that is for displaying in accordance with image data for the left eye (hereinafter, this image data is referred to as left-eye image data). The left-eye image data and the right-eye image data are for images which are to be viewed from different observation positions. The display image displayed in accordance with the right-eye image data is viewed with the right eye while the display image displayed in accordance with the left-eye image data is viewed with the left eye. In this way, the display image is recognized as a three-dimensional image.
On the other hand, when the image is displayed as a two-dimensional image, that is, when the voltage is applied on the liquid crystal layer 123, light having passed through the region 121A and light having passed through the region 121B are such that the optical axes thereof are symmetrically tilted from the transmission axis of the polarizer 115.
That is, both of the light having passed through the regions 121A and the regions 121B pass through the polarizer with the same transmittance, thereby not effectuating the function of the parallax barrier. Thus, the image from all the pixels are viewed with the both right and left eyes, thereby allowing the display of the two-dimensional image.
Whether the parallax barrier is effectuated or not is switched over by whether or not applying the voltage on the liquid crystal layer 123 of the switching liquid crystal panel 102, thereby switching over between the two-dimensional image display and the three-dimensional image display.
The liquid crystal display apparatus for dual-use in displaying the two-dimensional image and the three-dimensional image is arranged such that the voltages to be applied on the liquid crystal layers 113 and the 123 of the dot-matrix type liquid crystal panel 101 and the switching liquid crystal panel 102 are inverted in polarity in order to prevent deterioration of display quality. Control circuits respectively controls timing of the polarity inversion of the dot-matrix type liquid crystal panel 101 and the switching liquid crystal panel 102.
Further, a power source circuit for generating the voltage to be supplied to the dot-matrix type liquid crystal panel 101 has a different configuration from that of a power source circuit for generating the voltage to be supplied to the switching liquid crystal panel 102.
In the conventional arrangements, however, no attention is paid to a cycle of the timing of the polarity inversion of the switching liquid crystal panel 102. Thus, the timing of the polarity inversion is not in synchronism with a vertical cycle of the dot-matrix type liquid crystal panel 101. Moreover, in some cases, the polarity inversion cycle is extremely shorter than the vertical cycle of the dot-matrix type liquid crystal panel 101.
The inventors of the present invention noted the significance of the timing of the polarity inversion of the switching liquid crystal panel 102, and a problem associated with the timing of the polarity inversion: the timing may cause display quality deterioration. This problem is explained below.
A position of a polarity inversion point of the switching liquid crystal panel 102 with respect to a vertical display starting point is changed if the timing of the polarity inversion of the switching liquid crystal panel 102 is not in synchronism with the vertical cycle of the dot-matrix type liquid crystal panel 101.
Moreover, the polarity of the switching liquid crystal panel 102 is inverted in plural times within an effective display period of the dot-matrix type liquid crystal panel 101, if the polarity inversion cycle of the switching liquid crystal panel 102 is extremely shorter than the vertical cycle.
FIG. 14 illustrates waveforms of the voltage applied on the dot-matrix type liquid crystal panel 101 and the switching liquid crystal panel 102 in the conventional arrangement. In FIG. 14, the upper waveform represents the vertical display start signal of the dot-matrix type liquid crystal panel 101. The middle waveform is a waveform of the voltage applied on the pixel electrode 114 of the dot-matrix type liquid crystal panel 101. The lower waveform is a waveform of the voltage applied on the switching liquid crystal panel 102.
As illustrated in FIG. 14, one vertical period has the effective display period and blanking period: the voltage is applied on the pixel electrodes on 1 to N lines (i.e. 1 to N rows) in the effective display period; and the blanking period is a rest of the one vertical period. In FIG. 14, the polarity of the switching liquid crystal panel 102 is inverted four times in the effective display period.
Moreover, it can be understood from FIG. 14 that the polarity inversion of the switching liquid crystal panel 102 in accordance with the vertical display start signal occurs at different points in respective vertical periods.
The inventors of the present inventions found out that, due to capacity coupling occurred between the upper electrode 122 of the switching liquid crystal panel 102 and pixel electrode 114 of the dot-matrix type liquid crystal panel 101, an video signal to be inputted in the pixel electrode 114 is fluctuated in accordance with the inversion of the voltage applied on the switching liquid crystal panel 102, the fluctuation taking place at the timing of polarity inversion of the voltage applied on the switching liquid crystal panel 102.
FIG. 15 illustrates a waveform of the video signal when the voltage applied on the switching liquid crystal panel 102 is inverted.
As illustrated in FIG. 15, at a timing a at which the voltage applied on the switching liquid crystal panel 102 (i.e. the voltage between the upper electrode 122 and the lower electrode 124) is inverted from the negative polarity to the positive polarity, the video signal is shifted toward a positive direction. On the other hand, as a timing b at which the voltage is inverted from the positive polarity to the negative polarity, the video signal is shifted toward a negative direction.
Therefore, a pixel electrode whose active element is ON at the timing a is excessively charged when the video signal is a signal shifted to the positive direction. On the other hand, this pixel electrode is not sufficiently charged when the video signal is a signal shifted to the negative direction. Moreover, a pixel electrode whose active element is ON at the timing b is not sufficiently charged when the video signal is a signal shifted to the positive direction. On the other hand, this pixel electrode is excessively charged when the video signal is a signal shifted to the negative direction.
The excess charging on the pixel electrode causes a dark line on the displayed screen. Moreover, the insufficient charging on the pixel electrode causes a bright line on the displayed screen.
Therefore, if the polarity of the switching liquid crystal panel 102 is inverted in plural times within one effective display period as described above, a plurality of bright lines or dark lines appear, causing display deterioration. Further, in case where the polarity inversion point of the inversion carried out in accordance with the vertical display start signal is changed per vertical period, the bright line or the dark line flows on the screen.
The inventors of the present invention found the problem associated with the polarity inversion of the voltage applied on another liquid crystal panel in the display apparatus configured by assembling a matrix-type liquid crystal panel with the another display panel. The problem is that the polarity inversion causes fluctuation in the voltage applied on the pixel electrode, thereby causing a severe deterioration of display quality.